The invention relates to an apparatus and method for continuously stretching a web of thermoplastic sheet into an arcuate form of desired curvature for use in laminated windshields.
For the production of automobile windshields, in particular those with a tinted band along the upper side, it is desirable to stretch the originally straight web of, for example, polyvinylbutyral interlayer into a curved web that fits along the upper side of the windshield. This curved web is subsequently cut to produce the appropriate cut shaped blanks that will be used as interlayer between two sheets of glass for the manufacture of laminated windshields.
This stretching operation is done in an installation, commonly known as a "shaper", where the polyvinylbutyral web is first heated and then pulled onto either solid conical drums or onto an adjustable multi-spoke pyramid, sometimes referred to as the "umbrella" technology.
In the case of conical drums as, for example, disclosed in U.S. Pat. No. 5,071,601, several sets of drums are needed for obtaining different radii of the edges of the blanks. This implies a high number of change overs and will affect contamination levels and line capacity.
This "umbrella" technology is described in, for example, U.S. Pat. Nos. 2,933,759 and 3,696,186. The imprintations of the spokes on the sheet are usually negligible and are not considered as a problem.
This technology is particularly advantageous as the shape, i.e. the radius of curvature of the edges, may be easily varied by mechanically adjusting the opening of the umbrella, even while running.
According to the prior art, the hot polyvinylbutyral sheet is kept some time in its stretched form on the pyramid to allow the new shape to set. The setting is an exponential function that converges to 100% setting but will never attain this number.
After an acceptable setting has been achieved, the polyvinylbutyral sheet is cooled down while still on the pyramid to avoid distortion of the set shape when subsequently handling the hot soft sheet.
An undesired side effect of the cooling down is that it also freezes the remaining snap back in the sheet, which will be freed during the subsequent storage and during the lamination process.
It is an object of this invention to make available a method for producing shaped blanks having significantly improved dimensional stability.
It is another object of this invention to make available a shaping unit which can serve to produce shaped blanks having significantly improved dimensional stability.
The above and other objects can now be achieved with the aid of technology whereby the shaped blanks are first subjected to hot-setting followed by relaxation (or snapback) and subsequent cooling. In a preferred manner, such process is executed with the aid of at least two truncated (adjustable) multispoke pyramids.